Angiogenesis, also called neovascularization, involves the formation of sprouts from preexistent blood vessels and their invasion into surrounding tissue. A related process, vasculogenesis, involves the differentiation of endothelial cells and angioblasts that are already present throughout a tissue, and their subsequent linking together to form blood vessels.
Angiogenesis occurs extensively during development, and also occurs in the healthy body during wound healing in order to restore blood flow to tissues after injury or insult. Angiogenesis, however, has also been implicated in cancer and tumor formation. Indeed, the quantity of blood vessels in a tumor tissue is a strong negative prognostic indicator in breast cancer (Weidner et al., (1992) J. Natl. Cancer Inst. 84:1875-1887), prostate cancer (Weidner et al., (1993) Am. J. Pathol. 143:401-409), brain tumors (Li et al., (1994) Lancet 344:82-86), and melanoma (Foss et al., (1996) Cancer Res. 56:2900-2903). Angiogenesis has also recently been implicated in other disease states in many areas of medicine, including rheumatology, dermatology, cardiology and ophthalmology. In particular, undesirable or pathological tissue-specific angiogenesis has been associated with certain specific disease states including rheumatoid arthritis, atherosclerosis, and psoriasis (see e.g., Fan et al., (1995) Trends Pharmacol. Sci. 16: 57; and Folkman (1995) Nature Med. 1: 27). Furthermore, the alteration of vascular permeability is thought to play a role in both normal and pathological physiological processes (Cullinan-Bove et al., (1993) Endocrinol. 133: 829; Senger et al., (1993) Cancer and Metastasis Reviews 12: 303). Although the angiogenic process in each of these diseases is likely to share many features with developmental angiogenesis and tumor angiogenesis, each may also have unique aspects conferred by the influence of surrounding cells.
Several ocular disorders involve alterations in angiogenesis. For example, diabetic retinopathy, the third leading cause of adult blindness (accounting for almost 7% of blindness in the USA), is associated with extensive angiogenic events. Nonproliferative retinopathy is accompanied by the selective loss of pericytes within the retina, and their loss results in dilation of associated capillaries dilation and a resulting increase in blood flow. In the dilated capillaries, endothelial cells proliferate and form outpouchings, which become microaneurysms, and the adjacent capillaries become blocked so that the area of retina surrounding these microaneurysms is not perfused. Eventually, shunt vessels appear between adjacent areas of micro aneurysms, and the clinical picture of early diabetic retinopathy with micro aneurysms and areas of nonperfused retina is seen. The microaneurysms leak and capillary vessels may bleed, causing exudates and hemorrhages. Once the initial stages of background diabetic retinopathy are established, the condition progresses over a period of years, developing into proliferative diabetic retinopathy and blindness in about 5% of cases. Proliferative diabetic retinopathy occurs when some areas of the retina continue losing their capillary vessels and become nonperfused, leading to the appearance of new vessels on the disk and elsewhere on the retina. These new blood vessels grow into the vitreous and bleed easily, leading to preretinal hemorrhages. In advanced proliferative diabetic retinopathy, a massive vitreous hemorrhage may fill a major portion of the vitreous cavity. In addition, the new vessels are accompanied by fibrous tissue proliferation that can lead to traction retinal detachment.
Diabetic retinopathy is associated primarily with the duration of diabetes mellitus; therefore, as the population ages and diabetic patients live longer, the prevalence of diabetic retinopathy will increase. Laser therapy is currently used in both nonproliferative and proliferative diabetic retinopathy. Focal laser treatment of the leaking microaneurysms surrounding the macular area reduces visual loss in 50% of patients with clinically significant macular edema. In proliferative diabetic retinopathy, panretinal photocoagulation results in several thousand tiny burns scattered throughout the retina (sparing the macular area); this treatment reduces the rate of blindness by 60 percent. Early treatment of macular edema and proliferative diabetic retinopathy prevents blindness for 5 years in 95% of patients, whereas late treatment prevents blindness in only 50 percent. Therefore, early diagnosis and treatment are essential.
Another ocular disorder involving neovascularization is age-related macular degeneration (AMD), a disease that affects approximately one in ten Americans over the age of 65. AMD is characterized by a series of pathologic changes in the macula, the central region of the retina, which is accompanied by decreased visual acuity, particularly affecting central vision. AMD involves the single layer of cells called the retinal pigment epithelium that lies immediately beneath the sensory retina. These cells nourish and support the portion of the retina in contact with them, i.e., the photoreceptor cells that contain the visual pigments. The retinal pigment epithelium lies on the Bruch membrane, a basement membrane complex which, in AMD, thickens and becomes sclerotic. New blood vessels may break through the Bruch membrane from the underlying choroid, which contains a rich vascular bed. These vessels may in turn leak fluid or bleed beneath the retinal pigment epithelium and also between the retinal pigment epithelium and the sensory retina. Subsequent fibrous scarring disrupts the nourishment of the photoreceptor cells and leads to their death, resulting in a loss of central visual acuity. This type of age-related maculopathy is called the “wet” type because of the leaking vessels and the subretinal edema or blood. The wet type accounts for only 10% of age-related maculopathy cases but results in 90% of cases of legal blindness from macular degeneration in the elderly. The “dry” type of age-related maculopathy involves disintegration of the retinal pigment epithelium along with loss of the overlying photoreceptor cells. The dry type reduces vision but usually only to levels of 20/50 to 20/100.
AMD is accompanied by distortion of central vision with objects appearing larger or smaller or straight lines appearing distorted, bent, or without a central segment. In the wet type of AMD, a small detachment of the sensory retina may be noted in the macular area, but the definitive diagnosis of a subretinal neovascular membrane requires fluorescein angiography. In the dry type, drusen may disturb the pigmentation pattern in the macular area. Drusen are excrescences of the basement membrane of the retinal pigment epithelium that protrude into the cells, causing them to bulge anteriorly; their role as a risk factor in age-related maculopathy is unclear. No treatment currently exists for the dry type of age-related maculopathy. Laser treatment is used in the wet type of age-related maculopathy and initially obliterates the neovascular membrane and prevents further visual loss in about 50% of patients at 18 months. By 60 months, however, only 20% still have a substantial benefit.
Multiple molecular mediators of angiogenesis have been identified including basic and acidic fibroblast growth factors (aFGF, bFGF), transforming growth factors alpha and beta (TGFα, TGFβ), platelet-derived growth factor (PDGF), angiogenin, platelet-derived endothelial cell growth factor (PD-ECGF), interleukin-8 (IL-8), and vascular endothelial growth factor (VEGF). Other stimulators implicated in angiogenesis include angiopoietin-1, Del-1, follistatin, granulocyte colony-stimulating factor (G-CSF), hepatocyte growth factor (HGF), leptin, midkine, placental growth factor, pleiotrophin (PTN), progranulin, proliferin, and tumor necrosis factor-alpha (TNF-alpha). In addition, control of angiogenesis is further mediated by a number of negative regulators of angiogenesis produced by the body including angioarrestin, angiostatin (plasminogen fragment), antiangiogenic antithrombin III, cartilage-derived inhibitor (CDI), CD59 complement fragment, endostatin (collagen XVIII fragment), fibronectin fragment, gro-beta, heparinases, heparin hexasaccharide fragment, human chorionic gonadotropin (hCG), interferon alpha/beta/gamma, interferon inducible protein (IP-10), interleukin-12, kringle 5 (plasminogen fragment), metalloproteinase inhibitors (TIMPs), 2-methoxyestradiol, placental ribonuclease inhibitor, plasminogen activator inhibitor, platelet factor-4 (PF4), prolactin 16 kD fragment, proliferin-related protein (PRP), retinoids, tetrahydrocortisol-S, thrombospondin-1 (TSP-1), vasculostatin, and vasostatin (calreticulin fragment).
Among these angiogenic regulators, VEGF appears to play a key role as a positive regulator of the abnormal angiogenesis accompanying tumor growth (reviewed in Brown et al., (1996) Control of Angiogenesis (Goldberg and Rosen, eds.) Birkhauser, Basel, and Thomas (1996) J. Biol. Chem. 271:603-606). Furthermore, recently the role of the PDGF-B member of the PDGF family of signaling molecules has been under investigation, since it appears to play a role in the formation, expansion and proper function of perivascular cells, sometimes referred to as mural cells, e.g., vascular smooth muscle, mesangial cells, and pericytes.
While much has been learned about angiogenesis, or neovascularization, accompanying development, wound healing and tumor formation, it remains to be determined whether there are differences between these forms of angiogenesis and ocular angiogenesis. Significantly, while angiogenesis accompanying, e.g., collateral blood vessel formation in the heart, may be beneficial and adaptive to the organism, pathological ocular neovascularization accompany, e.g., AMD, has no known benefit and often leads to blindness (for review, see Campochiaro (2000) J. Cell. Physiol. 184: 301-10). Therefore, although advances in the understanding of the molecular events accompanying neovascularization have been made, there exists a need to utilize this understanding to develop further methods for treating neovascular diseases disorders, including ocular neovascular diseases and disorders such as the choroidal neovascularization that occurs with AMD and diabetic retinopathy.